rustdoc: remove unused CSS #main-content > .line-numbers

This selector was added in 10b9370. It
became unreachable when 09150f8 made it so
that `.line-numbers` are always nested below `.example-wrap`, even on
source pages.

Author: notriddle

src/librustdoc/html/static/css/rustdoc.css

@@ -1917,10 +1917,6 @@ in storage.js plus the media query with (min-width: 701px)
 		border-bottom: 1px solid;
 	}
 
-	#main-content > .line-numbers {
-		margin-top: 0;
-	}
-
 	.notable-traits .notable-traits-tooltiptext {
 		left: 0;
 		top: 100%;